1. Field of the Invention
The present invention relates to a state information communications system for sending the state information indicating the operating state of each device between interconnected terminal units linked via a communications bus, and also to a state information communications method.
2. Description of the Prior Art
A conventional state information communications system on a domestic digital bus (D2B) (IEC TC (84)-(S)861,2 Draft--Domestic Digital Bus (D2B)) used for data communications between, for example, audio and video equipment is shown in FIG. 1.
Each terminal unit, such as 7, comprises a communications controller 2a, terminal device 5a, and device information memory 4a. The terminal unit 7 is, for example, a VCR and the terminal unit 8 is, for example, a television receiver. The terminal device 5a includes a tape deck, a tape loading mechanism, a tuner, etc. When a terminal unit 7 is polled for its state information by another terminal unit 8 on the network, the communications controller 2a fetches the requested state information from the device buffer 21a in the communications controller 2a and replies by sending the information over the communications line. The state information stored in the device buffer 21a is supplied from the device information memory 4a which is continuously updated in real-time as the actual terminal state of the terminal device 5a changes, for example, from stop state to play state.
With this method, however, when the play command is provided to terminal unit 7 from terminal unit 8, and thereafter terminal unit 7 is polled for its state information by another terminal unit 8, terminal unit 7 replies that its state is STOP until the terminal device 5a is completely changed to the PLAY state. Thus, during a state change operation, terminal unit 8 frequently (such as after every 400 ms) requests terminal unit 7 to produce the state information, and terminal unit 7 produces STOP state information as many time as the number of requests produced from the terminal unit 8, resulting in increase of the network traffic through the bus line 1.
Furthermore, during the state change terminal unit 8 is not informed whether the state in terminal unit 7 is really changing or it is not operating because of some defect, such as machine failure, or tape not being loaded.